The cytotoxicity associated with the lipid peroxidation is due in part to the conjugation of amino acid residues in protein with 4-hydroxy-2- nonenal which is a product of lipid peroxidation. A highly sensitive immunochemical method was developed for the detection and quantitation of 4-hydroxynonenal (HNE)-protein conjugates. With this method, it was demonstrated that HNE-protein conjugates are produced when crude homogenates of rat liver are treated with HNE or are subjected to oxidative stresses obtained by exposure to tert-butylhydroperoxide or to metal-ion-catalyzed oxidation systems.